This invention relates to improvements in a tire molding apparatus used in a shaping and vulcanizing press and more particularly to a unique steam-chambered tire mold for use in standard steam dome presses.
For more than three decades, major tire manufacturers have employed presses, such as those sold under the name "Bag-O-Matic" (McNeil) or "Autoform" (NRM) which combine shaping and vulcanization of a previously formed pneumatic tire in one operation in accordance with the so-called Soderquist system. Presses of this type are disclosed, for example in U.S. Pat. Nos. 2,358,762; 2,358,764; 2,775,789 and 2,808,618. The heat for curing may be provided internally from the curing bladder or diaphragm and externally from a steam dome or steam-heated platens.
In a typical steam dome press, such as that of U.S. Pat. No. 2,358,762 or U.S. Pat. No. 2,775,789, the curing bladder or diaphragm is collapsed to permit a green tire to be positioned on the lower mold section, the press is then closed and the curing bag bladder expanded by steam forcing the tire into the mold. During the curing period, high pressure steam or high temperature water is provided in the curing bladder until the cure is completed. The length of the curing cycle depends on the size or mass of the tire being cured and may be from about 30 to 90 minutes (min) for a heavy duty truck tire. At the completion of the cure, the press automatically opens. The curing bladder may be completely extended mechanically and collapsed by application of vacuum to permit removal of the tire from the mold.
The steam dome presses built thirty years ago were designed to provide curing temperatures and pressures which were then considered optimal. The presses could operate with dome steam pressures up to 75 pounds per square inch gage ("psig"). These older presses must be operated using limited internal steam or water pressures typically in the range from about 200 to 250 psig. More recently modern shaping and vulcanizing presses have been designed to operate at much higher pressures and higher curing temperatures. The internal water pressures have been increased to 350-400 psig or higher to improve the quality of the tires, and higher curing temperatures have been employed to shorten the curing time.
Unfortunately the old steam dome presses cannot operate safely in such manner. Many of these presses were originally designed for operation at pressures of 75 psig and are today operated at dome pressures of around 70 psig which corresponds to a steam temperature of less than 160.degree. C. The deterioration of the old presses requires lowering of the maximum steam pressure within the dome and also limits the internal water or steam pressure as both contribute to press-opening force. The internal water or steam pressure in the curing bladder must, for safety reasons, be less than the pressure desired for optimum tire quality.
Pressure vessel safety regulations on many standard steam dome presses prevent use of dome steam pressures substantially in excess of 75 psig and make it impossible to employ the high curing temperatures which are preferred for commercial production. The longer curing time substantially increases the cost of manufacture of the tires.
Steam dome presses have other disadvantages. They are energy inefficient because of leakage problems, the large mass being heated, and the large quantities of dome steam lost each time the press is opened. Also, because of the large mass being heated, temperature response is slow, making it nearly impossible to tailor temperature gradients or to profile the external mold temperature during the cure cycle. These and the other disadvantages described above have been recognized in the tire industry for decades, but the old steam dome presses have been retained in spite of their limitations particularly because of the high cost of new equipment.
The domes of old steam dome presses are similar to those of standard platen presses (see U.S. Pat. No. 2,808,618), and platens can be added to such steam dome presses. However, conversion of the old presses to platen presses is expensive and not a satisfactory solution to the above problem of slow temperature response. The old steam dome presses are desirable for curing some large tires where lower curing temperatures are required and where it is important to provide uniform heat to the outer circumferential portions of the tire mold (i.e. molds having a wide tread). Platen presses are less desirable because the heat comes solely from the top and bottom of the mold and because the heat transfer across the metal-to-metal interface is often uneven. These are some of the reasons why almost all large or heavy duty tires for trucks, tractors and heavy construction vehicles are cured in steam dome presses rather than platen presses. Tire manufacturers prefer to keep their old steam dome presses and to add newer ones which are rated at pressures up to 100 psig.
Unfortunately the high cost of modern steam dome presses limits their number and they have serious limitations. Pressure vessel safety regulations prevent operation of such presses at steam pressures in excess of 100 psig and therefore prevent use of steam temperatures in excess of 170.degree. C. This places serious limitations on the cure time cycle reductions.
The rubber tire industry has had to rely heavily on its old equipment for manufacture of heavy duty truck tires and other large tires. Prior to the present invention tire manufacturers did not know how to make full utilization of the existing steam dome presses or how to overcome the temperature and pressure limitations dictated by pressure vessel safety regulations and slow mold temperature response to externally applied temperature conditions.